Electron Paramagnetic Resonance as a Probe of Technologically Relevant Processing Effects on CdTe Solar Cell Materials

Michael Pigott, Liam Young, Patrick M. Lenahan

Pennsylvania State University, University Park, PA, 16802, USA

Adam Halverson, Kristian W. Andreini, Bas A. Korevaar

G.E. Global Research, Niskayuna, NY, 12309, USA

Abstract — The performance of polycrystalline thin film CdTe solar cells is limited by as yet poorly understood deep level defects which serve as recombination centers. Electron paramagnetic resonance (EPR) has unrivaled analytical power and sensitivity in the identification of deep level defects in semiconductors; however, very little EPR literature exists with regard to technologically relevant processing of present day polycrystalline CdTe solar cells. In this study we explore the effects of several features important in these present day processing techniques: (1) effects of CdCl2 etching and (2) effects of Cu on the CdTe. Cu is widely used with CdTe solar cells and is thought to play a significant role in CdTe solar cells performance. Although the EPR spectra in all the cases we explored are dominated by a substitutional Mn site signal, our results are clearly sensitive to multiple changes caused by the aforementioned processing parameters. Index Terms — CdTe, thin film, deep level defects,

recombination centers, EPR.

I. INTRODUCTION

The Shockley-Queisser Limit for CdTe based solar cells is about 30%. The present day performance limit of these devices is considerably lower; the current record efficiency is 18.3% [1]. A dominating factor in this reduced efficiency is the presence of deep level defects which serve as recombination centers. Unfortunately, the physical and chemical nature of these centers is not yet well understood. Electron paramagnetic resonance (EPR) has a sensitivity of about 1010 total defect density within the sample and analytical power to identify CdTe deep level defects. In fact, a significant number of EPR studies have already been carried out [2]-[7]. However, very little in the CdTe EPR literature deals with the effects of technologically meaningful processing parameters. In this study we utilize EPR to investigate the effect of CdCl2 treatment and the introduction of Cu into CdTe.

EPR spectroscopy, places a sample in a large, slowly varying magnetic field while simultaneously exposing the sample to microwave frequency electromagnetic radiation. In the simplest of cases, EPR takes place when the resonance condition (1) is satisfied:

Hghv β= (1).

Here h is Planck’s constant, ν is microwave frequency, β is the Bohr magneton, and H is the field of resonance. The g is, for

a single defect orientation within the magnetic field, a number typically close to 2. It depends upon the orientation of the defect within the magnetic field and is often expressed as a second rank tensor. The resonance condition for a perfectly isolated electron would be satisfied for g = 2.00232. In essentially all systems, deviations from this free electron value are observed; the deviations are due to spin orbit coupling. In many defect systems a second important contribution to the resonance condition results from interactions between electrons and the magnetic moments of nuclei. These so called hyperfine interactions can also provide convincing and often detailed information about the physical and chemical nature of paramagnetic defect sites as well as provide information about defect electron wave functions. Again, in quite simple cases with only a single magnetic nucleus, the hyperfine interactions alter the resonance condition to that of expression (2):

AmHghv I+= β (2),

where h, ν, g, β, and H are defined as in expression (1) and mI represents the nuclear spin quantum number and A is an orientation dependent hyperfine parameter, also generally expressed as a second rank tensor. A nuclear spin of I can have nuclear spin quantum number, mI, of –I, -(I-1), -(I-2), …, (I-2), (I-1), I. Thus, a nucleus with a spin of 1/2 can exhibit mI

= -1/2 and +1/2. A nucleus with spin 5/2 can exhibit mI = -5/2, -3/2, -1/2, 1/2, 3/2, 5/2. Generally 2I+1 local fields and thus 2I+1 resonance lines are generated by the presence of a single nucleus of spin I. Often more than one magnetic nucleus and/or more than one defect orientation exists within the sample under study. It is generally a straightforward process to extend the expressions (1) and (2) to these more complex cases [8]-[9].

EPR results are typically presented as the first derivative of the absorption signal. The x-axis is the magnetic field (Gauss) of the EPR spectrum and y-axis is in arbitrary units [8]-[9].

II. EXPERIMENTAL TECHNIQUES

Our measurements utilized X Band Bruker Biospin EPR spectrometers and a Janis Helium Cryostat. All involved EPR measurements were taken at 5 K. All samples were polycrystalline CdTe powders.

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III. EXPERIMENTAL RESULTS

Figure 1 shows the EPR results taken on fourat 5 K. The samples are labeled: CdTe, CdTe+Cu, and CdTe+CdCl2+Cu. The labelsexplanatory. The CdTe+CdCl2 sample was subjected to a CdCl2 treatment step. The CdTe+Cu samples were exposed to Cu. The CdTe+CdCl2+Cu samples were exposed to both CdCl2 and Cu in consecutive steps.

Fig. 1. EPR spectra of four CdTe samples. All measurements were made at 5 K.

Figure 1 shows that the EPR traces of all the samples are dominated by six strong lines in the central part of the scans. In all four samples, the six lines are about the same size and have a g value near the free electron g (2.0023)CdTe samples, the measured six line spectrum g=2.006. The measured hyperfine splitting is approximately 61 Gauss.six-line spectrum is consistent with a near spin 5/2 nucleus. There are relatively few atoms 100% abundant spin 5/2 nucleus. Al, Mn, Ionly four elements that meet these requirementsMn the most plausible. An earlier study by Schwartz dealing with the Mn in CdTe reports an isotropic 2.00597 and a hyperfine splitting of 61 Gaussentirely consistent with results presented in this paperTherefore, with a high degree of certainty, we associate the six line spectrum with Mn. Although the spectra are dominated by a Mn associated defect, of much greater interest are

ESULTS

four CdTe samples d: CdTe, CdTe+CdCl2,

The labels are self-sample was subjected to a

step. The CdTe+Cu samples were exposed to +Cu samples were exposed to both

. All measurements were

Figure 1 shows that the EPR traces of all the samples are dominated by six strong lines in the central part of the scans.

are about the same size and the free electron g (2.0023). In all the

CdTe samples, the measured six line spectrum g=2.006. The is approximately 61 Gauss. The

line spectrum is consistent with a near 100% abundant There are relatively few atoms with a near

. Al, Mn, I, and Pr are the only four elements that meet these requirements, with Al and

by Schwartz et al. reports an isotropic g value of

of 61 Gauss [6], results irely consistent with results presented in this paper.

Therefore, with a high degree of certainty, we associate the six line spectrum with Mn. Although the spectra are dominated by

of much greater interest are the subtle

sample to sample variations resulting from the differences in processing.

In Figures 2, 3, and 4 we compare the CdTe+CdClCdTe+Cu and CdTe+CdCl2+Cu spectra respectively to that of CdTe. In these figures we have normalized the amplitudes based on the dominating 6 line spectrum. We normalized the amplitudes in order to compensate for slight differences in sample volume. These comparisons clearly show that these processing steps have effects on the EPR spectrum.

Fig. 2. EPR spectra of CdTe compared to CdTe+CdClMeasurements were made at 5 K.

Figure 2 clearly shows that the CdClsmall reduction in the amplitude of the wide background signal that encompasses most of the scan and interferes with the 4th Mn peak. The CdCl2 treatment also introduces two additional peaks at 3173 G and 3633 G.

The comparison illustrated in Figure 3 does not gross differences between the simple CdTe and CdTe+Cusave for a change around g=2.002, indicating that the Cu by itself does not introduce high densities of any new paramagnetic defects.

Figure 4 illustrates very significant differences between the simple CdTe and the CdTe+CdClalso indicates a sharpening of the spectra at 3173 G and 3633 G. Several other large differences are also noticeable

resulting from the differences in

In Figures 2, 3, and 4 we compare the CdTe+CdCl2 and +Cu spectra respectively to that of

CdTe. In these figures we have normalized the amplitudes based on the dominating 6 line spectrum. We normalized the

o compensate for slight differences in sample volume. These comparisons clearly show that these processing steps have effects on the EPR spectrum.

compared to CdTe+CdCl2.

shows that the CdCl2 treatment causes a small reduction in the amplitude of the wide background signal that encompasses most of the scan and interferes with

treatment also introduces two additional peaks at 3173 G and 3633 G.

e comparison illustrated in Figure 3 does not show any een the simple CdTe and CdTe+Cu,

save for a change around g=2.002, indicating that the Cu by itself does not introduce high densities of any new

ustrates very significant differences between the simple CdTe and the CdTe+CdCl2+Cu sample. This figure

he spectra at 3173 G and 3633 are also noticeable.

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Fig. 3. EPR spectra of CdTe compared to CdTe+Cu. Measurements were made at 5 K.

Fig. 4. EPR spectra of CdTe compared to CdTe+CMeasurements were made at 5 K.

compared to CdTe+Cu.

compared to CdTe+CdCl2+Cu.

In order to more clearly identify the

sample-to-sample differences we utilized the EPR simulation program Easy Spin [10] to dominating Mn signal. By subtracting this simulated Mn spectrum from the four traces, we largely, though still quite imperfectly, eliminated the dominating effectreveal more subtle effects of the processing variationAnother set of spectra, utilizing this spectral subtraction approach, is plotted in figure 5. Although the approximation we obtain in the spectral subtraction is not perfect, it effectively reduces the amplitudes of all the Mn peaksextent that additional EPR spectra are

Fig. 5. EPR spectra of the CdTe samples dominating Mn signal (imperfectly) indicate a secondary EPR signal with a g value near that of the free electron. Note the presence of the strongest and most complex underlying spectrum in the sample with both chlorine and copper.

One spectrum which is now clearly subtraction traces of figure 5 is a narrowelectron g with a value close to 2.002signal nearly doubles in both samples that contain Cuamplitude is approximately 80 percent bigger in Crelative to CdTe. This near doubling in amplitude indicate that the addition of Cu also causes the concentration of this defect to double. However, energy might also account for thisshift in Fermi level is very possible, as Cu acts as a dopant for CdTe. The CdCl2 appears to slightly reduce the

to more clearly identify the processing induced differences we utilized the EPR simulation

to closely approximate the . By subtracting this simulated Mn

spectrum from the four traces, we largely, though still quite eliminated the dominating effects of the Mn to

of the processing variations. a, utilizing this spectral subtraction

. Although the approximation we obtain in the spectral subtraction is not perfect, it

reduces the amplitudes of all the Mn peaks to the are clearly visible.

CdTe samples taken at 5 K with the removed. The red arrows

with a g value near that of the free Note the presence of the strongest and most complex

underlying spectrum in the sample with both chlorine and copper.

clearly evident in the spectral subtraction traces of figure 5 is a narrow line near the free

close to 2.002. The amplitude of this signal nearly doubles in both samples that contain Cu. The

80 percent bigger in CdTe+Cu CdTe. This near doubling in amplitude may

indicate that the addition of Cu also causes the concentration However, changes in the Fermi

energy might also account for this doubled amplitude. The el is very possible, as Cu acts as a dopant for

o slightly reduce the g=2.002

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signal. Its amplitude in the CdTe+CdCl2 sample is reduced by approximately 20 percent compared to CdTe.

A third feature of all four spectra is a very broad underlying spectrum several hundred Gauss wide. A plausible source of this very broad line is the Cd vacancy, which has a strongly anisotropic g tensor (gǁ = 1.9555 and g┴ = 2.143) [7]. In a polycrystalline sample, such a g tensor would yield a very broad line reasonably consistent with our admittedly crude observations [10]. With regard to the narrow line with a g close to that of the free electron: color center like defects typically produce an EPR signal with a nearly isotropic or isotropic g value near the free electron g. Such defects in CdTe translate to a Te vacancy which has a well-documented g=2.000 [7]. Unfortunately, the EPR signal for the Te vacancy is typically only 50 G wide, so such an assignment does not explain the very broad background signal.

Another defect studied previously with a g value near that of the free electron is Cd+. Cd+ is formed by the oxidation of CdTe during the annealing process when temperatures exceed 300 oC. Cd+ is characterized by one intense, sharp, isotropic signal with a g=2.0002 [3]. The Cd+ signal is only a few Gauss wide and is not consistent with the wide background signal associated with this unknown defect. It may however be responsible for the narrow g=2.0025 and 2.0031 lines observed in all of the samples. The modest discrepancy in g could presumably be associated with the limits on our admittedly crude spectral subtractions.

It is of some interest that by far the most complex EPR pattern is that of the CdTe+CdCl2+Cu sample. These results suggest complex interactions between CdCl2 and Cu. The other interesting features are the signals that appear at 3173 G and 3633 G when CdTe is treated with CdCl2. An interpretation of these EPR lines is currently not clear.

IV. CONCLUSIONS

The results of this preliminary study indicate that EPR has the analytical power and sensitivity to identify defect centers created in processing steps, which are technologically meaningful in CdTe photovoltaic manufacturing. Although more work needs to be done to conclusively link the structure and energy levels to the various EPR spectra, the results strongly suggest that EPR could be a powerful tool for technologists involved in optimizing CdTe solar cell processing.

REFERENCES

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[3] Padam GK, Gupta SK, “Electron-spin resonance and transmission electron microscopy studies of solution-grown CdTe thin films.” Applied Physics Letters 53(10). 1988.

[4] P. Christmann, B. K. Meyer, J. Kreissl, R. Schwarz, K. W. Benz, "Vanadium in cdte: An electron-paramagnetic-resonance study", Physical Review B, vol.53, no.7, pp. 3634-3637, Feb 1996.

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[6] Schwartz RN, Wang CC, Trivedi S, Jagannathan GV, Davidson FM, Boyd PR, Lee U, “Spectroscopic and photorefractive characterization of cadmium telluride crystals codoped with vanadium and manganese.” Physical Review B 55(23):15378-15381. 1997.

[7] Meyer BK, Hofmann DM, “Anion and Cation Vacancies in CdTe.” Applied Physics a-Materials Science & Processing 61(2). 213-215. 1995.

[8] Poole CP, Electron Spin Resonance: A Comprehensive Treatise on Experimental Techniques. New York, New York: John Wiley and Sons, 1983.

[9] Weil JA, Bolton JR, Wertz JE, Electron Paramagnetic Resonance. New York, New York: John Wiley and Sons, 1994.

[10] Easy Spin, a comprehensive software package for spectral simulation and analysis in EPR. J. Magn. Reson. 178(1), 42-55. 2006

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